It is understood that electric drive machines are in wide-spread use. The current for driving the electric drive machine is introduced, in this instance, via one or more pairs of sliding contacts via the commutator into the rotor winding (armature winding). In most cases, these sliding contacts are made of a sintered material, which predominantly includes copper and graphite components. These sliding contacts as well as the commutator are subject to wear during operation. Starting devices are typically designed for short-term operation, and are normally suitable for 30,000 to 60,000 switching cycles.
If the starting device is to reach a higher load and a larger number of operations, the sliding contacts have to have a uniform load for a maximum possible number of operations to be achieved. At the present time, arrangements are selected for systems having 4 or 6 sliding contacts in which the sliding contacts have an angular distance of 60° from one another. This means that, in a system having 6 sliding contacts, there is in each case an angle of 60° between the sliding contacts. In a system having 4 sliding contacts, for one, there is in each case one positive sliding contact and one negative sliding contact directly opposite, that is, at an angle of 180°, and a second pair of sliding contacts, which is in itself again distanced by 180° is arranged in such a way that this second sliding contact pair is offset with its negative sliding contact by 60° from the positive sliding contact of the first sliding contact pair.
Based on the use of a bar number of the commutator that is not divisible by the pole number of the stator for starter motors (electric drive machine), as a rule, 28 bars or 23 bars, the result is a different stress on the positive sliding contacts and the negative sliding contacts, since the sliding contact pair rotational direction takes up different positions in the case of connecting a conductor of the rotor winding and in the case of switching off a conductor of the rotor winding on the bars. This difference has the effect that the sliding contact pairs are stressed differently, or rather, for individual sliding contact pairs, a different load profile is created. For one rotational direction of the drive machine, individual sliding contacts are acted upon using a different current load when running onto a bar or when running off a bar. As a result, the carbon brushes age or wear to a different degree, so that the achievable service life is unnecessarily reduced because of the greater wear of individual sliding contacts.
Therefore, there exists the object of prolonging the achievable service life of a drive machine, of the type named above, by making the service life of the individual sliding contacts as equal as possible among themselves. Thus, it may happen, for example that, at the end of the service life of a passenger car, the starting device has also reached the end of its service life with respect to its mechanics. But if one looks at the sliding contacts, one will frequently determine that, of four sliding contacts used, only one of the sliding contacts has worn down in the scope provided, while under certain circumstances, other sliding contacts of the same starting device have worn down only by ⅓ of the provided wear length of the sliding contacts. Now, if the required number of operations, and with that, the required number of starts of a starter, rises because of an operation having frequent repeated starts of the internal combustion engine, one has to take care of an optimum distribution of the current load and the load peaks for the sliding contacts.
Another alternative, namely, the extension of the sliding contact that is especially stressed, or of all the sliding contacts, is unsuccessful, as a rule, because a greater diameter of the pole tube of the electric drive machine is not disposable. An optimum distribution of the current load and the load peaks for the sliding contacts may be achieved, especially in the case of armatures or rotors having wave winding and not having a whole number ratio of bar number and pole pair of the stator because of a particular distribution of the sliding contacts over the circumference of the commutator. In the case of sufficiently great overlapping of the sliding contacts with the bars, in order to obtain an ideal division of the load and the load peaks over the sliding contacts, we have found that the optimal arrangement of the sliding contacts is not at an equiangular distance of the sliding contacts to one another. Depending on the rotational direction, one or more of the sliding contacts has to be offset by about 1°, up to a value of a quotient of 360° and the bar number of the commutator in, or counter to the rotational direction, compared to the symmetrical values at the respective angles (quotient of 360° and the pole number).
This achieves minimizing the current load maxima in the individual sliding contacts, and also as low as possible a variation between the sliding contacts. The load peaks and the integral currents thus may be lowered up to 25%, and differences in the wear of the individual sliding contacts, known from endurance tests, may also be minimized. In individual starter systems, the equalization may mean differences in wear over the service life of up to 2 mm, which at the same time may mean up to more than 30% of the possible wear length of a sliding contact.